This invention relates to a lifting valve including a bellows to seal a valve rod passage.
Lifting valves of the generic type preferably are designed as shut-off valves and are used according. At this point, at least one component of the aperture and closing motions of the closing component is directed perpendicularly to the end surfaces, the seating surfaces. The passage of the valve rod actuating the closing component through the valve box is spanned by a bellows which concentrically encircles the valve rod. This bellows which may be formed as an expansion bellows (DE 32 15 799 C2), a corrugated tube or a diaphragm (EP 0 508 658 B1) extends, as a rule, from the closing component up to that portion of the valve box which is traversed by the valve rod.
Such a valve rod sealing by means of a bellows has the advantage that there is no sealing gap between the valve rod adapted to slide in an axial direction and the stationary valve box, into which a product carry-over can take place because of the motion of the valve rod relative to the valve box. It is known that such sealing gaps pose sanitary problem zones in which germ or bacteria formation may occur, which will then provoke reinfections of the product in the valve box because of the valve rod motion, the so-called "elevator effect".
Using a bellows to seal the passage of a valve rod is compulsory for high sanitary requirements especially in the aseptic range of the pharmaceutical chemical, or foodstoff-processing industries. From the aforementioned DE 32 15 799 C2, an aseptic valve has been known which has a closing component cooperating with a conical seating in a valve box with an appropriate annular sealing surface with the closing component being connected to a servo-drive via a screw and is joined to an expansion bellows coaxially encircling the screw and constituting the screw seating which, at its free end, has a connecting flange adapted to be locked in place between the valve box proper and a valve lantern with a conical sealing surface which bears against a complementary annular seating surface of the box surface which bears against a complementary annular seating surface of the box where the closing piece, the expansion bellows, and its connecting flange form a rotation-symmetrical replacement unit which is exchangeable as a whole and is open to the top. This so-called replacement unit is made of a polytetrafluoroethylene material (PTFE such as Teflon) which, as positive properties, exhibits high elasticity, resistance to chemicals, and a long service life while putting up with relatively large plastic ductility (heavy propension to "flow" or "creep").
The aforementioned replacement unit, amongst other things, has the two problem zones which now are indicated.
1. The connecting flange, along with its conical sealing surface, defines a sensible annular gap in an interaction with its complementary annular seating surface in the valve box. In order that this annular gap between the connecting flange and the stationary valve box parts bordering it on the other side permanently remain tight and do not "work" under varying conditions of operation (pressure and temperature) the connecting flange is squeezed between the valve box and a box closing component (box top), i.e. the connecting flange is kept under a bias in its locked position. It is known now that the material preferably used for the replacement unit at the beginning (PTFE) tends to exhibit a certain creep behaviour under varying pressures and temperatures, as seen over a prolonged period of time, which causes the shape of the connecting flange to slightly change. In operation, this causes the bias which prevents an expansion of the sealing gap and is produced by the squeezing action to gradually diminish and the sealing gap to "work". This might cause a product to penetrate in the "breathing" sealing gap with the aforementioned consequences of product carry-over and subsequent reinfection. PA1 2. The expansion bellows which has been known from DE 32 15 799 C2 is an integral part of the so-called replacement unit which may be exchanged in the valve, if required. To this end, it is necessary that the box aperture and, hence, the measure of the connecting flange which corresponds thereto be designed at least as large as the outer diameter of the closing component. As is shown in FIG. 2 of the aforementioned document the tensile forces acting onto the connecting flange if the expansion bellows is under a tensile stress and the reaction forces directed counter to these forces are not situated on a joint line of action at the locking point of the connecting flange, but are at a radial spacing from each other. This spacing and, hence, the bending moment produced by this couple of forces will even be increased, as a rule, because efforts are made to dimension the expansion bellows so as to be as small as possible for reasons of strength. This causes the point of application of the tensile force exerted by the expansion bellows onto the connecting flange to migrate further inwardly in a radial direction so that the aforementioned radial spacing between the two forces and, therefore, the bending moment will continue to increase.
The bending moment just discussed will now lead to the fact that the connecting flange squeezed between the box and the box closing component apart from being subjected to this squeezing stress, also experiences a bending stress which provokes an increased strain on the material and, hence, an intensified change in shape in this region. This aggravates the critical situation previously described with reference to the sealing gap under item 1.
In the document EP 0 508 658 B1 also mentioned at the beginning, in which an aseptic valve construction is described with a bellows of the generic type, the aforementioned problem existing in the region of the critical sealing gap on the conical sealing surface of the connecting flange was discussed as a subject already. To mitigate the problem encountered, a suggestion is made there that the complementary annular shoulder seating in the valve box which corresponds to the conical sealing surface of the connecting flange in the valve box should have a lip completely inside which is inclined downwardly from the inwardly and upwardly inclined annular surface of the valve box. Such a measure may possibly prevent the sealing gap from being expanded prematurely. However, it constitutes no long-lasting measure because if the bellows material creeps at this point the bias required will diminish as well and will not be permanently ensured by any further recognizable measures.
A satisfactory solution to this problem may be successful only if the influencing factors which were described as being adverse under items 1 and 2 above can be reduced and/or if they can be counteracted by appropriate measures.
The expansion bellows which has been known from DE 32 15 799 C2 encircles the valve rod at a relatively large radial clearance. Although the latter allows a view into and makes possible an intervention in the interior of the bellows in case of need and, thus, possibly favours a detection of a bellows defect on time it is rather infavourable with respect to the stability of the bellows because of an attack of the flow and squeezing forces from the space enclosing the bellows. A conditional remedy may be provided here either by decreasing the bellows diameter in the region of the bellows folds or increasing the rod diameter. The first action, however, leads to a larger stress on the box-end connecting flange of the expansion bellows at a bending moment, which stresses the critical gap between the conical sealing surface of the connecting flange and its complementary annular seating surface in the box in a dynamically alternating way. As a result of the existing propension of the bellows material to flow at a high product pressure and/or high product temperature, there is a risk of gap expansion and product penetration into this region with all of the adverse consequences known, amongst which are germ formation and product reinfection.
Leaving out attention to the aforementioned gap problems in the region of the connecting flange for a moment and directing attention exclusively to an increase in bellows stability in the region of the bellows folds such increase may be obtained by a decrease in the break between the bellows folds and the valve rod, either by reducing the bellows diameter or, if the bellows diameter is not changed, by a supporting action from the inside, e.g. by increasing the rod diameter or by retracting the supporting tube which tightly borders the bellows (DE 42 43 111 A1).
A supporting action from the inside has shown that if bellows materials are used which have a strong propension to undergo durable deformation, so-called "creeping", when under a stress, such as polytetrafluoroethylene, the bellows fold regions adjoining the rod come to increasingly bear against the rod or the supporting pipe because of this permanent deformation so that the respective fold interior which is bordered by the bellows fold, on one hand, and the rod or supporting pipe itself in the inside region, on the other, defines a self-contained cavern from which the medium enclosed therein cannot or can insufficiently escape. The medium enclosed can either be a check medium with which the interior of the bellows is filled and which can be detected via suitable devices, or leaking media which enter the interior of the bellows and result from a defect in the bellows (e.g. a product). This puts in question any dependable and up-to-date indication of a bellows defect or its instant detection.
As a result of the axial shift of the rod, there is a relative motion between the expansion bellows and the rod, which approaches the zero at the point connecting the bellows to the rod and has its maximum rate at the box-side end of the bellows. As the bellows folds bear against the rod this relative motion is impeded because there is a friction between the bellows fold and the rod, This can go so far that the latter does not occur at all in regions of a small relative motion and it is only the bellows folds in the vicinity of the box-side connection of the bellows which perform the entire shifting motion of the rod. As a result, stresses higher than those on which the design was based will occur in this region with the possible consequence being a premature rupture of the bellows and, thus, a reduction in the service life of the bellows.
It is the object of the present invention to ensure for a lifting valve a reliable, hygienically proper, and durable static sealing of the connecting flange of the closing-member unit in the valve box under the occurring forces at an adequately long service life. In addition, a reliable indication of a bellows defect is intended to be ensured throughout the full service life of the bellows even for a bellows which tightly encircles the valve rod.